Experience-dependent increase in CA1 place cell spatial information, but not spatial reproducibility, is dependent on the autophosphorylation of the alpha-isoform of the calcium/calmodulin-dependent protein kinase II

Adult neurogenesis improves spatial information encoding in the mouse hippocampus

Adult neurogenesis is a unique form of neuronal plasticity in which newly generated neurons are integrated into the adult dentate gyrus in a process that is modulated by environmental stimuli. Adult-born neurons can contribute to spatial memory, but it is unknown whether they alter neural representations of space in the hippocampus. Using in vivo two-photon calcium imaging, we find that male and female mice previously housed in an enriched environment, which triggers an increase in neurogenesis, have increased spatial information encoding in the dentate gyrus. Ablating adult neurogenesis blocks the effect of enrichment and lowers spatial information, as does the chemogenetic silencing of adult-born neurons. Both ablating neurogenesis and silencing adult-born neurons decreases the calcium activity of dentate gyrus neurons, resulting in a decreased amplitude of place-specific responses. These findings are in contrast with previous studies that suggested a predominantly inhibitory action for adult-born neurons. We propose that adult neurogenesis improves representations of space by increasing the gain of dentate gyrus neurons and thereby improving their ability to tune to spatial features. This mechanism may mediate the beneficial effects of environmental enrichment on spatial learning and memory.

A consistent map in the medial entorhinal cortex supports spatial memory

The medial entorhinal cortex (MEC) is hypothesized to function as a cognitive map for memory-guided navigation. How this map develops during learning and influences memory remains unclear. By imaging MEC calcium dynamics while mice successfully learned a novel virtual environment over ten days, we discovered that the dynamics gradually became more spatially consistent and then stabilized. Additionally, grid cells in the MEC not only exhibited improved spatial tuning consistency, but also maintained stable phase relationships, suggesting a network mechanism involving synaptic plasticity and rigid recurrent connectivity to shape grid cell activity during learning. Increased c-Fos expression in the MEC in novel environments further supports the induction of synaptic plasticity. Unsuccessful learning lacked these activity features, indicating that a consistent map is specific for effective spatial memory. Finally, optogenetically disrupting spatial consistency of the map impaired memory-guided navigation in a well-learned environment. Thus, we demonstrate that the establishment of a spatially consistent MEC map across learning both correlates with, and is necessary for, successful spatial memory.

A consistent map in the medial entorhinal cortex supports spatial memory

The medial entorhinal cortex (MEC) is hypothesized to function as a cognitive map for memory-guided navigation. How this map develops during learning and influences memory remains unclear. By imaging MEC calcium dynamics while mice successfully learned a novel virtual environment over ten days, we discovered that the dynamics gradually became more spatially consistent and then stabilized. Additionally, grid cells in the MEC not only exhibited improved spatial tuning consistency, but also maintained stable phase relationships, suggesting a network mechanism involving synaptic plasticity and rigid recurrent connectivity to shape grid cell activity during learning. Increased c-Fos expression in the MEC in novel environments further supports the induction of synaptic plasticity. Unsuccessful learning lacked these activity features, indicating that a consistent map is specific for effective spatial memory. Finally, optogenetically disrupting spatial consistency of the map impaired memory-guided navigation in a well-learned environment. Thus, we demonstrate that the establishment of a spatially consistent MEC map across learning both correlates with, and is necessary for, successful spatial memory.

Synaptic memory and CaMKII

Ca2+/calmodulin-dependent protein kinase II (CaMKII) and long-term potentiation (LTP) were discovered within a decade of each other and have been inextricably intertwined ever since. However, like many marriages, it has had its up and downs. Based on the unique biochemical properties of CaMKII, it was proposed as a memory molecule before any physiological linkage was made to LTP. However, as reviewed here, the convincing linkage of CaMKII to synaptic physiology and behavior took many decades. New technologies were critical in this journey, including in vitro brain slices, mouse genetics, single-cell molecular genetics, pharmacological reagents, protein structure, and two-photon microscopy, as were new investigators attracted by the exciting challenge. This review tracks this journey and assesses the state of this marriage 40 years on. The collective literature impels us to propose a relatively simple model for synaptic memory involving the following steps that drive the process: 1) Ca2+ entry through N-methyl-d-aspartate (NMDA) receptors activates CaMKII. 2) CaMKII undergoes autophosphorylation resulting in constitutive, Ca2+-independent activity and exposure of a binding site for the NMDA receptor subunit GluN2B. 3) Active CaMKII translocates to the postsynaptic density (PSD) and binds to the cytoplasmic C-tail of GluN2B. 4) The CaMKII-GluN2B complex initiates a structural rearrangement of the PSD that may involve liquid-liquid phase separation. 5) This rearrangement involves the PSD-95 scaffolding protein, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and their transmembrane AMPAR-regulatory protein (TARP) auxiliary subunits, resulting in an accumulation of AMPARs in the PSD that underlies synaptic potentiation. 6) The stability of the modified PSD is maintained by the stability of the CaMKII-GluN2B complex. 7) By a process of subunit exchange or interholoenzyme phosphorylation CaMKII maintains synaptic potentiation in the face of CaMKII protein turnover. There are many other important proteins that participate in enlargement of the synaptic spine or modulation of the steps that drive and maintain the potentiation. In this review we critically discuss the data underlying each of the steps. As will become clear, some of these steps are more firmly grounded than others, and we provide suggestions as to how the evidence supporting these steps can be strengthened or, based on the new data, be replaced. Although the journey has been a long one, the prospect of having a detailed cellular and molecular understanding of learning and memory is at hand.

Active experience, not time, determines within-day representational drift in dorsal CA1

Memories of past events can be recalled long after the event, indicating stability. But new experiences are also integrated into existing memories, indicating plasticity. In the hippocampus, spatial representations are known to remain stable but have also been shown to drift over long periods of time. We hypothesized that experience, more than the passage of time, is the driving force behind representational drift. We compared the within-day stability of place cells' representations in dorsal CA1 of the hippocampus of mice traversing two similar, familiar tracks for different durations. We found that the more time the animals spent actively traversing the environment, the greater the representational drift, regardless of the total elapsed time between visits. Our results suggest that spatial representation is a dynamic process, related to the ongoing experiences within a specific context, and is related to memory update rather than to passive forgetting.

Organization of hippocampal CA3 into correlated cell assemblies supports a stable spatial code

Hippocampal subfield CA3 is thought to stably store memories in assemblies of recurrently connected cells functioning as a collective. However, the collective hippocampal coding properties that are unique to CA3 and how such properties facilitate the stability or precision of the neural code remain unclear. Here, we performed large-scale Ca2+ imaging in hippocampal CA1 and CA3 of freely behaving mice that repeatedly explored the same, initially novel environments over weeks. CA3 place cells have more precise and more stable tuning and show a higher statistical dependence with their peers compared with CA1 place cells, uncovering a cell assembly organization in CA3. Surprisingly, although tuning precision and long-term stability are correlated, cells with stronger peer dependence exhibit higher stability but not higher precision. Overall, our results expose the three-way relationship between tuning precision, long-term stability, and peer dependence, suggesting that a cell assembly organization underlies long-term storage of information in the hippocampus.

Experience-dependent changes in hippocampal spatial activity and hippocampal circuit function are disrupted in a rat model of Fragile X Syndrome

BACKGROUND

Fragile X syndrome (FXS) is a common single gene cause of intellectual disability and autism spectrum disorder. Cognitive inflexibility is one of the hallmarks of FXS with affected individuals showing extreme difficulty adapting to novel or complex situations. To explore the neural correlates of this cognitive inflexibility, we used a rat model of FXS (Fmr1^-/y).

METHODS

We recorded from the CA1 in Fmr1^-/y and WT littermates over six 10-min exploration sessions in a novel environment-three sessions per day (ITI 10 min). Our recordings yielded 288 and 246 putative pyramidal cells from 7 WT and 7 Fmr1^-/y rats, respectively.

RESULTS

On the first day of exploration of a novel environment, the firing rate and spatial tuning of CA1 pyramidal neurons was similar between wild-type (WT) and Fmr1^-/y rats. However, while CA1 pyramidal neurons from WT rats showed experience-dependent changes in firing and spatial tuning between the first and second day of exposure to the environment, these changes were decreased or absent in CA1 neurons of Fmr1^-/y rats. These findings were consistent with increased excitability of Fmr1^-/y CA1 neurons in ex vivo hippocampal slices, which correlated with reduced synaptic inputs from the medial entorhinal cortex. Lastly, activity patterns of CA1 pyramidal neurons were dis-coordinated with respect to hippocampal oscillatory activity in Fmr1^-/y rats.

LIMITATIONS

It is still unclear how the observed circuit function abnormalities give rise to behavioural deficits in Fmr1^-/y rats. Future experiments will focus on this connection as well as the contribution of other neuronal cell types in the hippocampal circuit pathophysiology associated with the loss of FMRP. It would also be interesting to see if hippocampal circuit deficits converge with those seen in other rodent models of intellectual disability.

CONCLUSIONS

In conclusion, we found that hippocampal place cells from Fmr1^-/y rats show similar spatial firing properties as those from WT rats but do not show the same experience-dependent increase in spatial specificity or the experience-dependent changes in network coordination. Our findings offer support to a network-level origin of cognitive deficits in FXS.

Postsynaptic burst reactivation of hippocampal neurons enables associative plasticity of temporally discontiguous inputs

A fundamental unresolved problem in neuroscience is how the brain associates in memory events that are separated in time. Here, we propose that reactivation-induced synaptic plasticity can solve this problem. Previously, we reported that the reinforcement signal dopamine converts hippocampal spike timing-dependent depression into potentiation during continued synaptic activity (Brzosko et al., 2015). Here, we report that postsynaptic bursts in the presence of dopamine produce input-specific LTP in mouse hippocampal synapses 10 min after they were primed with coincident pre- and post-synaptic activity (post-before-pre pairing; Δt = -20 ms). This priming activity induces synaptic depression and sets an NMDA receptor-dependent silent eligibility trace which, through the cAMP-PKA cascade, is rapidly converted into protein synthesis-dependent synaptic potentiation, mediated by a signaling pathway distinct from that of conventional LTP. This synaptic learning rule was incorporated into a computational model, and we found that it adds specificity to reinforcement learning by controlling memory allocation and enabling both 'instructive' and 'supervised' reinforcement learning. We predicted that this mechanism would make reactivated neurons activate more strongly and carry more spatial information than non-reactivated cells, which was confirmed in freely moving mice performing a reward-based navigation task.

Multiple-Timescale Representations of Space: Linking Memory to Navigation

When navigating through space, we must maintain a representation of our position in real time; when recalling a past episode, a memory can come back in a flash. Interestingly, the brain's spatial representation system, including the hippocampus, supports these two distinct timescale functions. How are neural representations of space used in the service of both real-world navigation and internal mnemonic processes? Recent progress has identified sequences of hippocampal place cells, evolving at multiple timescales in accordance with either navigational behaviors or internal oscillations, that underlie these functions. We review experimental findings on experience-dependent modulation of these sequential representations and consider how they link real-world navigation to time-compressed memories. We further discuss recent work suggesting the prevalence of these sequences beyond hippocampus and propose that these multiple-timescale mechanisms may represent a general algorithm for organizing cell assemblies, potentially unifying the dual roles of the spatial representation system in memory and navigation.

Role of Ca2+/Calmodulin-Dependent Protein Kinase Type II in Mediating Function and Dysfunction at Glutamatergic Synapses

Glutamatergic synapses harbor abundant amounts of the multifunctional Ca²⁺/calmodulin-dependent protein kinase type II (CaMKII). Both in the postsynaptic density as well as in the cytosolic compartment of postsynaptic terminals, CaMKII plays major roles. In addition to its Ca²⁺-stimulated kinase activity, it can also bind to a variety of membrane proteins at the synapse and thus exert spatially restricted activity. The abundance of CaMKII in glutamatergic synapse is akin to scaffolding proteins although its prominent function still appears to be that of a kinase. The multimeric structure of CaMKII also confers several functional capabilities on the enzyme. The versatility of the enzyme has prompted hypotheses proposing several roles for the enzyme such as Ca²⁺ signal transduction, memory molecule function and scaffolding. The article will review the multiple roles played by CaMKII in glutamatergic synapses and how they are affected in disease conditions.

Bias-free estimation of information content in temporally sparse neuronal activity

Applying information theoretic measures to neuronal activity data enables the quantification of neuronal encoding quality. However, when the sample size is limited, a naïve estimation of the information content typically contains a systematic overestimation (upward bias), which may lead to misinterpretation of coding characteristics. This bias is exacerbated in Ca2+ imaging because of the temporal sparsity of elevated Ca2+ signals. Here, we introduce methods to correct for the bias in the naïve estimation of information content from limited sample sizes and temporally sparse neuronal activity. We demonstrate the higher accuracy of our methods over previous ones, when applied to Ca2+ imaging data recorded from the mouse hippocampus and primary visual cortex, as well as to simulated data with matching tuning properties and firing statistics. Our bias-correction methods allowed an accurate estimation of the information place cells carry about the animal's position (spatial information) and uncovered the spatial resolution of hippocampal coding. Furthermore, using our methods, we found that cells with higher peak firing rates carry higher spatial information per spike and exposed differences between distinct hippocampal subfields in the long-term evolution of the spatial code. These results could be masked by the bias when applying the commonly used naïve calculation of information content. Thus, a bias-free estimation of information content can uncover otherwise overlooked properties of the neural code.

Place Cells in Head-Fixed Mice Navigating a Floating Real-World Environment

The hippocampal place cell system in rodents has provided a major paradigm for the scientific investigation of memory function and dysfunction. Place cells have been observed in area CA1 of the hippocampus of both freely moving animals, and of head-fixed animals navigating in virtual reality environments. However, spatial coding in virtual reality preparations has been observed to be impaired. Here we show that the use of a real-world environment system for head-fixed mice, consisting of an air-floating track with proximal cues, provides some advantages over virtual reality systems for the study of spatial memory. We imaged the hippocampus of head-fixed mice injected with the genetically encoded calcium indicator GCaMP6s while they navigated circularly constrained or open environments on the floating platform. We observed consistent place tuning in a substantial fraction of cells despite the absence of distal visual cues. Place fields remapped when animals entered a different environment. When animals re-entered the same environment, place fields typically remapped over a time period of multiple days, faster than in freely moving preparations, but comparable with virtual reality. Spatial information rates were within the range observed in freely moving mice. Manifold analysis indicated that spatial information could be extracted from a low-dimensional subspace of the neural population dynamics. This is the first demonstration of place cells in head-fixed mice navigating on an air-lifted real-world platform, validating its use for the study of brain circuits involved in memory and affected by neurodegenerative disorders.

LTD is involved in the formation and maintenance of rat hippocampal CA1 place-cell fields

Hippocampal synaptic plasticity includes both long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength, and has been implicated in shaping place field representations that form upon initial exposure to a novel environment. However, direct evidence causally linking either LTP or LTD to place fields remains limited. Here, we show that hippocampal LTD regulates the acute formation and maintenance of place fields using electrophysiology and blocking specifically LTD in freely-moving rats. We also show that exploration of a novel environment produces a widespread and pathway specific de novo synaptic depression in the dorsal hippocampus. Furthermore, disruption of this pathway-specific synaptic depression alters both the dynamics of place field formation and the stability of the newly formed place fields, affecting spatial memory in rats. These results suggest that activity-dependent synaptic depression is required for the acquisition and maintenance of novel spatial information.

Phospholipase Cβ3 in the hippocampus may mediate impairment of memory by long-term blockade of orexin 1 receptors assessed by the Morris water maze

Orexin-A is an endogenous peptide with receptors throughout the brain. According to some recent research, learning and memory are affected by the central administration of orexin; however, no study so far has investigated the long-term inhibition of the orexinergic system. The present study has evaluated the effect of pretraining administration of orexin 1 receptor (OXR1) antagonist, SB-334867, on the acquisition of memory. The Morris water maze (MWM) task was used for training and trial purposes in all groups. Memory performance was analyzed by measuring escape latency, traveled distance, and time spent in the target quadrant. Moreover, the effect of SB-334867 on phospholipase Cβ3 (PLCβ3) levels in the CA1 region of hippocampus slices was examined. Hippocampus slices were prepared using an immunohistochemistry (IHC) approach. SB-334867 (20 mg/kg) increased escape latency in SB-treated rats compared to SB-vehicle group (P < 0.01). SB-treated rats spent less time in the target quadrant compared to the SB-vehicle group (P < 0.001). Distance traveled in the target quadrant was significantly more in SB-treated rats compared to the SB-vehicle group (P < 0.001). Furthermore, SB-334867 decreased PLCβ3 levels in the CA1 of the hippocampus (P < 0.01 and P < 0.05, respectively). Put together, our results suggest that the long-term inhibition of OXR1 plays a prominent role in spatial learning and memory, probably by attenuating PLCβ3 in CA1 neurons.

The interplay between somatic and dendritic inhibition promotes the emergence and stabilization of place fields

During the exploration of novel environments, place fields are rapidly formed in hippocampal CA1 neurons. Place cell firing rate increases in early stages of exploration of novel environments but returns to baseline levels in familiar environments. Although similar in amplitude and width, place fields in familiar environments are more stable than in novel environments. We propose a computational model of the hippocampal CA1 network, which describes the formation, dynamics and stabilization of place fields. We show that although somatic disinhibition is sufficient to form place fields, dendritic inhibition along with synaptic plasticity is necessary for place field stabilization. Our model suggests that place cell stability can be attributed to strong excitatory synaptic weights and strong dendritic inhibition. We show that the interplay between somatic and dendritic inhibition balances the increased excitatory weights, such that place cells return to their baseline firing rate after exploration. Our model suggests that different types of interneurons are essential to unravel the mechanisms underlying place field plasticity. Finally, we predict that artificially induced dendritic events can shift place fields even after place field stabilization.

SK Channel Modulates Synaptic Plasticity by Tuning CaMKIIα/β Dynamics

N-Methyl-D-Aspartate Receptor 1 (NMDAR)-linked Ca++ current represents a significant percentage of post-synaptic transient that modulates synaptic strength and is pertinent to dendritic spine plasticity. In the hippocampus, Ca++ transient produced by glutamatergic ionotropic neurotransmission facilitates Ca++-Calmodulin-dependent kinase 2 (CaMKII) Thr286 phosphorylation and promote long-term potentiation (LTP) expression. At CA1 post-synaptic densities, Ca++ transients equally activate small conductance (SK2) channel which regulates excitability by suppressing Ca++ movement. Here, we demonstrate that upstream attenuation of GluN1 function in the hippocampus led to a decrease in Thr286 CaMKIIα phosphorylation, and increased SK2 expression. Consistent with the loss of GluN1 function, potentiation of SK channel in wild type hippocampus reduced CaMKIIα expression and abrogate synaptic localization of T286 pCaMKIIα. Our results demonstrate that positive modulation of SK channel at hippocampal synapses likely refine GluN1-linked plasticity by tuning dendritic localization of CaMKIIα.

Dendritic mechanisms of hippocampal place field formation

Place cells in the hippocampus are thought to form a cognitive map of space and a memory of places. How this map forms when animals are exposed to novel environments has been the subject of a great deal of research. Numerous technical advances over the past decade greatly increased our understanding of the precise mechanisms underlying place field formation. In particular, it is now possible to connect cellular and circuit mechanisms of integration, firing, and plasticity discovered in brain slices, to processes taking place in vivo as animals learn and encode novel environments. Here, we focus on recent results and describe the dendritic mechanisms most likely responsible for the formation of place fields. We also discuss key open questions that are likely to be answered in the coming years.

Stress and the Etiopathogenesis of Alzheimer's Disease and Depression

Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder with a complex physiopathology whose initiators are poorly defined. Accumulating clinical and experimental evidence suggests a causal role of lifetime stress in AD. This chapter summarizes current knowledge about how chronic stress and its accompanying high levels of glucocorticoid (GC) secretion, trigger the two main pathomechanisms of AD: (i) misprocessing of amyloid precursor protein (APP) and the generation of amyloid beta (Aβ) and (ii) Tau hyperphosphorylation and aggregation. Given that depression is a well-known stress-related illness, and the evidence that depression may precede AD, this chapter also explores neurobiological mechanisms that may be common to depressive and AD pathologies. This review also discusses emerging insights into the role of Tau and its malfunction in disrupting neuronal cascades and neuroplasticity and, thus triggering brain pathology.

Impaired spatial memory codes in a mouse model of Rett syndrome

The Mecp2^+/- mouse model recapitulates many phenotypes of patients with Rett syndrome (RTT), including learning and memory deficits. It is unknown, however, how the disease state alters memory circuit functions in vivo in RTT mice. Here we recorded from hippocampal place cells, which are thought to encode spatial memories, in freely moving RTT mice and littermate controls. We found that place cells in RTT mice are impaired in their experience-dependent increase of spatial information. This impairment is accompanied by an enhanced baseline firing synchrony of place cells within ripple oscillations during rest, which consequently occludes the increase in synchrony after a novel experience. Behaviorally, contextual memory is normal at short but not long time scale in RTT mice. Our results suggest that hypersynchrony interferes with memory consolidation and leads to impaired spatial memory codes in RTT mice, providing a possible circuit mechanism for memory deficits in Rett Syndrome.

Inducible Knockout of the Cyclin-Dependent Kinase 5 Activator p35 Alters Hippocampal Spatial Coding and Neuronal Excitability

p35 is an activating co-factor of Cyclin-dependent kinase 5 (Cdk5), a protein whose dysfunction has been implicated in a wide-range of neurological disorders including cognitive impairment and disease. Inducible deletion of the p35 gene in adult mice results in profound deficits in hippocampal-dependent spatial learning and synaptic physiology, however the impact of the loss of p35 function on hippocampal in vivo physiology and spatial coding remains unknown. Here, we recorded CA1 pyramidal cell activity in freely behaving p35 cKO and control mice and found that place cells in the mutant mice have elevated firing rates and impaired spatial coding, accompanied by changes in the temporal organization of spiking both during exploration and rest. These data shed light on the role of p35 in maintaining cellular and network excitability and provide a physiological correlate of the spatial learning deficits in these mice.

Hippocampal Remapping and Its Entorhinal Origin

The activity of hippocampal cell ensembles is an accurate predictor of the position of an animal in its surrounding space. One key property of hippocampal cell ensembles is their ability to change in response to alterations in the surrounding environment, a phenomenon called remapping. In this review article, we present evidence for the distinct types of hippocampal remapping. The progressive divergence over time of cell ensembles active in different environments and the transition dynamics between pre-established maps are discussed. Finally, we review recent work demonstrating that hippocampal remapping can be triggered by neurons located in the entorhinal cortex.

Increased Prevalence of Calcium Transients across the Dendritic Arbor during Place Field Formation

Hippocampal place cell ensembles form a cognitive map of space during exposure to novel environments. However, surprisingly little evidence exists to support the idea that synaptic plasticity in place cells is involved in forming new place fields. Here we used high-resolution functional imaging to determine the signaling patterns in CA1 soma, dendrites, and axons associated with place field formation when mice are exposed to novel virtual environments. We found that putative local dendritic spikes often occur prior to somatic place field firing. Subsequently, the first occurrence of somatic place field firing was associated with widespread regenerative dendritic events, which decreased in prevalence with increased novel environment experience. This transient increase in regenerative events was likely facilitated by a reduction in dendritic inhibition. Since regenerative dendritic events can provide the depolarization necessary for Hebbian potentiation, these results suggest that activity-dependent synaptic plasticity underlies the formation of many CA1 place fields.

Systemic administration of two different anxiolytic drugs decreases local field potential theta frequency in the medial entorhinal cortex without affecting grid cell firing fields

Neurons coding spatial location (grid cells) are found in medial entorhinal cortex (MEC) and demonstrate increasing size of firing fields and spacing between fields (grid scale) along the dorsoventral axis. This change in grid scale correlates with differences in theta frequency, a 6-10Hz rhythm in the local field potential (LFP) and rhythmic firing of cells. A relationship between theta frequency and grid scale can be found when examining grid cells recorded in different locations along the dorsoventral axis of MEC. When describing the relationship between theta frequency and grid scale, it is important to account for the strong positive correlation between theta frequency and running speed. Plotting LFP theta frequency across running speeds dissociates two components of this relationship: slope and intercept of the linear fit. Change in theta frequency through a change in the slope component has been modeled and shown experimentally to affect grid scale, but the prediction that change in the intercept component would not affect grid scale has not been tested experimentally. This prediction about the relationship of intercept to grid scale is the primary hypothesis tested in the experiments presented here. All known anxiolytic drugs decrease hippocampal theta frequency despite their differing mechanisms of action. Specifically, anxiolytics decrease the intercept of the theta frequency-running speed relationship in the hippocampus. Here we demonstrate that anxiolytics decrease the intercept of the theta frequency-running speed relationship in the MEC, similar to hippocampus, and the decrease in frequency through this change in intercept does not affect grid scale.

Impaired hippocampal place cell dynamics in a mouse model of the 22q11.2 deletion

Hippocampal place cells represent the cellular substrate of episodic memory. Place cell ensembles reorganize to support learning but must also maintain stable representations to facilitate memory recall. Despite extensive research, the learning-related role of place cell dynamics in health and disease remains elusive. Using chronic two-photon Ca²⁺ imaging in hippocampal area CA1 of wild-type and Df(16)A^+/- mice, an animal model of 22q11.2 deletion syndrome, one of the most common genetic risk factors for cognitive dysfunction and schizophrenia, we found that goal-oriented learning in wild-type mice was supported by stable spatial maps and robust remapping of place fields toward the goal location. Df(16)A^+/- mice showed a significant learning deficit accompanied by reduced spatial map stability and the absence of goal-directed place cell reorganization. These results expand our understanding of the hippocampal ensemble dynamics supporting cognitive flexibility and demonstrate their importance in a model of 22q11.2-associated cognitive dysfunction.

Experience-dependent shaping of hippocampal CA1 intracellular activity in novel and familiar environments

The hippocampus is critical for producing stable representations of familiar spaces. How these representations arise is poorly understood, largely because changes to hippocampal inputs have not been measured during spatial learning. Here, using intracellular recording, we monitored inputs and plasticity-inducing complex spikes (CSs) in CA1 neurons while mice explored novel and familiar virtual environments. Inputs driving place field spiking increased in amplitude – often suddenly – during novel environment exploration. However, these increases were not sustained in familiar environments. Rather, the spatial tuning of inputs became increasingly similar across repeated traversals of the environment with experience – both within fields and throughout the whole environment. In novel environments, CSs were not necessary for place field formation. Our findings support a model in which initial inhomogeneities in inputs are amplified to produce robust place field activity, then plasticity refines this representation into one with less strongly modulated, but more stable, inputs for long-term storage.

DOI:http://dx.doi.org/10.7554/eLife.23040.001

Conditional Knockout of Cav2.1 Disrupts the Accuracy of Spatial Recognition of CA1 Place Cells and Spatial/Contextual Recognition Behavior

Hippocampal pyramidal neurons play an essential role in processing spatial information as implicated with its place-dependent firing. Although, previous slice physiology studies have reported that voltage gated calcium channels contribute to spike shapes and corresponding firing rate in the hippocampus, the roles of P/Q type calcium channels (Cav2.1) underlying neural activity in behaving mice have not been well-investigated. To determine physiological and behavioral roles of Cav2.1, we conducted place cell recordings in CA1 and hippocampus dependent learning/memory tasks using mice lacking Cav2.1 in hippocampal pyramidal neurons under CamK2α-Cre recombinase expression. Results suggested that impairments shown in behavioral tasks requiring spatial and contextual information processing were statistically significant while general neurological behaviors did not differ between groups. In particular, deficits were more profound in recognition than in acquisition. Furthermore, place cell recordings also revealed that the ability to recollect spatial representation on re-visit in the conditional knockout was also altered in terms of the cue recognition while the capability of a place cell to encode a place was intact compared to the control group. Interestingly, CA1 pyramidal neurons of conditional knockout mice showed reduced burst frequency as well as abnormal temporal patterns of burst spiking. These results provide potential evidence that Cav2.1 in hippocampal pyramidal cells modulates temporal integration of bursts, which, in turn, might influence the recognition of place field and consequently disrupt spatial recognition ability.

Place cells and long-term potentiation in the hippocampus

Place cells show location-specific firing patterns according to an animal's position in an environment and are thought to contribute to the spatial representation required for self-navigation. Decades of study have extensively characterized the properties of place cells and suggested the involvement of long-term potentiation (LTP), a long-lasting synaptic strengthening, in place cell activity. Here, we review the basic characteristics of place cell activity and the findings that support the idea that LTP contributes to the formation, maintenance, and plasticity of place cell activity.

Slow stabilization of concurrently acquired hippocampal context representations

Hippocampal neurons exhibit spatially localized firing patterns that, at the population level, represent a particular environment or context. Many studies have examined how hippocampal neurons switch from an existing representation to a new one when the environment is changed, a process referred to as remapping. New representations were commonly thought to emerge rapidly, within a few minutes and then remain remarkably stable thereafter. However, a number of recent studies suggest that hippocampal representations may be more fluid than previously thought and most of the previous studies only required that subjects switch from a familiar environment to a novel one. In the present study, we examined the concurrent development of two distinct hippocampal representations by exposing rats to two distinct environmental contexts in an ABAB pattern and we recorded neuronal activity for eight daily training sessions. Hippocampal neurons exhibited normal place fields with typical firing properties during the initial exposure to each context on the first day. However, when the rats were returned to the original context after having spent 15 min in the second context, many of the neurons fired in new locations (i.e., they remapped) as if the rat had encountered a new environment. By the third day, the representations had stabilized and were highly consistent across visits to the same context. These results suggest that when subjects concurrently encode multiple contexts, hippocampal representations may require repeated experiences to fully stabilize. © 2016 Wiley Periodicals, Inc.

Theta Rhythmic Clock-Like Activity of Single Units in the Mouse Hippocampus

Theta rhythmic clock-like activity was observed in a small group of hippocampal CA1 neurons in freely behaving mice. These neurons were only persistently activated during theta states of waking exploration and rapid eye movement sleep, but were almost silent during the non-theta state of slow-wave sleep. Interestingly, these cells displayed a theta clock-like simple-spike firing pattern, and were capable of firing one spike per theta cycle during theta states. This is the first report of a unique class of hippocampal neurons with a clock-like firing pattern at the theta rhythm. We speculate that these cells may act as a temporal reference to participate in the theta-related temporal coding in the hippocampus.

SIGNIFICANCE STATEMENT

Theta oscillations, as the predominant rhythms in the hippocampus during waking exploration and rapid eye movement sleep, may be critical for temporal coding/decoding of neuronal information, and theta-phase precession in hippocampal place cells is one of the best demonstrations of such temporal coding. Here, we show that a unique small class of hippocampal CA1 neurons fired with a theta rhythmic clock-like firing pattern during theta states. These firing characteristics support the notion that these neurons may play a critical role in theta-related temporal coding in the hippocampus.

Spatial cognition in mice and rats: similarities and differences in brain and behavior

The increasing use of mice models in cognitive tasks that were originally designed for rats raises crucial questions about cross-species comparison in the study of spatial cognition. The present review focuses on the major neuroethological differences existing between mice and rats, with particular attention given to the neurophysiological basis of space coding. While little difference is found in the basic properties of space representation in these two species, it appears that the stability of this representation changes more drastically over time in mice than in rats. We consider several hypotheses dealing with attentional, perceptual, and genetic aspects and offer some directions for future research that might help in deciphering hippocampal function in learning and memory processes. WIREs Cogn Sci 2016, 7:406-421. doi: 10.1002/wcs.1411 For further resources related to this article, please visit the WIREs website.

Transient optogenetic inactivation of the medial entorhinal cortex biases the active population of hippocampal neurons

The mechanisms that enable the hippocampal network to express the appropriate spatial representation for a particular circumstance are not well understood. Previous studies suggest that the medial entorhinal cortex (MEC) may have a role in reproducibly selecting the hippocampal representation of an environment. To examine how ongoing MEC activity is continually integrated by the hippocampus, we performed transient unilateral optogenetic inactivations of the MEC while simultaneously recording place cell activity in CA1. Inactivation of the MEC caused a partial remapping in the CA1 population without diminishing the degree of spatial tuning across the active cell assembly. These changes remained stable irrespective of intermittent disruption of MEC input, indicating that while MEC input is integrated over long time scales to bias the active population, there are mechanisms for stabilizing the population of active neurons independent of the MEC. We find that MEC inputs to the hippocampus shape its ongoing activity by biasing the participation of the neurons in the active network, thereby influencing how the hippocampus selectively represents information.

Altered hippocampal information coding and network synchrony in APP-PS1 mice

β-amyloid is hypothesized to harm neural function and cognitive abilities by perturbing synaptic transmission and plasticity in Alzheimer's disease (AD). To assess the impact of this pathology on hippocampal neurons' ability to encode flexibly environmental information across learning, we performed electrophysiological recordings of CA1 hippocampal unit activity in AD transgenic mice as they acquired an action-reward association in a spatially defined environment; the behavioral task enabled the precise timing of discrete and intentional behaviors of the animal. We found that the proportion of behavioral task-sensitive cells in wild-type (WT) mice typically increased, whereas the proportion of place cells decreased with learning. In AD mice, this learning-dependent change of cell-discharge patterns was absent, and cells exhibited similar firings from the beginning to firings attained at the late learning stage in wild-type cells. These inflexible hippocampal representations of task and space throughout learning are accompanied by remarkable alterations of local oscillatory activity in the theta and ultra-fast ripple frequencies as well as learning abilities. The present data offer new insights into the in vivo cellular and network processes by which β-amyloid and other AD mutations may exert its harmful effects to produce cognitive and behavioral impairments in early stage of AD.

Progressive functional impairments of hippocampal neurons in a tauopathy mouse model

The age-dependent progression of tau pathology is a major characteristic of tauopathies, including Alzheimer's disease (AD), and plays an important role in the behavioral phenotypes of AD, including memory deficits. Despite extensive molecular and cellular studies on tau pathology, it remains to be determined how it alters the neural circuit functions underlying learning and memory in vivo. In rTg4510 mice, a Tau-P301L tauopathy model, hippocampal place fields that support spatial memories are abnormal at old age (7-9 months) when tau tangles and neurodegeneration are extensive. However, it is unclear how the abnormality in the hippocampal circuit function arises and progresses with the age-dependent progression of tau pathology. Here we show that in young (2-4 months of age) rTg4510 mice, place fields of hippocampal CA1 cells are largely normal, with only subtle differences from those of age-matched wild-type control mice. Second, high-frequency ripple oscillations of local field potentials in the hippocampal CA1 area are significantly reduced in young rTg4510 mice, and even further deteriorated in old rTg4510 mice. The ripple reduction is associated with less bursty firing and altered synchrony of CA1 cells. Together, the data indicate that deficits in ripples and neuronal synchronization occur before overt deficits in place fields in these mice. The results reveal a tau-pathology-induced progression of hippocampal functional changes in vivo.

Hippocampal place cell responses to distal and proximal cue manipulations in dopamine D2 receptor-knockout mice

The human hippocampus is critical for learning and memory. In rodents, hippocampal pyramidal neurons fire in a location-specific manner and form relational representations of environmental cues. The important roles of dopaminergic D1 receptors in learning and in hippocampal neural synaptic plasticity in novel environments have been previously shown. However, the roles of D2 receptors in hippocampal neural plasticity in response to novel and familiar spatial stimuli remain unclear. In order to clarify this issue, we recorded from hippocampal neurons in dopamine D2 receptor-knockout (D2R-KO) mice and their wild-type (WT) littermates during manipulations of distinct spatial cues in familiar and novel environments. Here, we report that D2R-KO mice showed substantial deficits in place-cell properties (number of place cells, intra-field firing rates, spatial tuning, and spatial coherence). Furthermore, although place cells in D2R-KO mice responded to manipulations of distal and proximal cues in both familiar and novel environments in a manner that was similar to place cells in WT mice, place fields were less stable in the D . The axes represent the differences between the peak and the valley of each waveform of EL2 and EL3.2R-KO mice in the familiar environment, but not in the novel environment. The present results suggested that D2 receptors in the hippocampus are important for place response stability. The place-cell properties of D2R-KO mice were similar to aged animals, suggesting that the alterations of place-cell properties in aged animals might be ascribed partly to alterations in the D2R in the HF of aged animals.

Dynamic NMDAR-mediated properties of place cells during the object place memory task

N-methyl-D-aspartate receptors (NMDAR) in the hippocampus participate in encoding and recalling the location of objects in the environment, but the ensemble mechanisms by which NMDARs mediate these processes have not been completely elucidated. To address this issue, we examined the firing patterns of place cells in the dorsal CA1 area of the hippocampus of mice (n = 7) that performed an object place memory (OPM) task, consisting of familiarization (T1), sample (T2), and choice (T3) trials, after systemic injection of 3-[(±)2-carboxypiperazin-4yl]propyl-1-phosphate (CPP), a specific NMDAR antagonist. Place cell properties under CPP (CPP–PCs) were compared to those after control saline injection (SAL–PCs) in the same mice. We analyzed place cells across the OPM task to determine whether they signaled the introduction or movement of objects by NMDAR-mediated changes of their spatial coding. On T2, when two objects were first introduced to a familiar chamber, CPP–PCs and SAL–PCs showed stable, vanishing or moving place fields in addition to changes in spatial information (SI). These metrics were comparable between groups. Remarkably, previously inactive CPP–PCs (with place fields emerging de novo on T2) had significantly weaker SI increases than SAL–PCs. On T3, when one object was moved, CPP–PCs showed reduced center-of-mass (COM) shift of their place fields. Indeed, a subset of SAL–PCs with large COM shifts (>7 cm) was largely absent in the CPP condition. Notably, for SAL–PCs that exhibited COM shifts, those initially close to the moving object followed the trajectory of the object, whereas those far from the object did the opposite. Our results strongly suggest that the SI changes and COM shifts of place fields that occur during the OPM task reflect key dynamic properties that are mediated by NMDARs and might be responsible for binding object identity with location.

Development of schemas revealed by prior experience and NMDA receptor knock-out

Prior experience accelerates acquisition of novel, related information through processes like assimilation into mental schemas, but the underlying neuronal mechanisms are poorly understood. We investigated the roles that prior experience and hippocampal CA3 N-Methyl-D-aspartate receptor (NMDAR)-dependent synaptic plasticity play in CA1 place cell sequence encoding and learning during novel spatial experiences. We found that specific representations of de novo experiences on linear environments were formed on a framework of pre configured network activity expressed in the preceding sleep and were rapidly, flexibly adjusted via NMDAR-dependent activity. This prior experience accelerated encoding of subsequent experiences on contiguous or isolated novel tracks, significantly decreasing their NMDAR-dependence. Similarly, de novo learning of an alternation task was facilitated by CA3 NMDARs; this experience accelerated subsequent learning of related tasks, independent of CA3 NMDARs, consistent with a schema-based learning. These results reveal the existence of distinct neuronal encoding schemes which could explain why hippocampal dysfunction results in anterograde amnesia while sparing recollection of old, schema-based memories.

DOI:http://dx.doi.org/10.7554/eLife.01326.001

Learning is an inherent feature of the living animals. During development and childhood we learn a large repertoire of items that we encounter for the first time in our life, such as the names of things and how to perform certain tasks. This de novo learning process takes a relatively long time and generally requires repeated exposures to the particular features of the external world that are being learned. Later in life, when we are exposed to novel but related features, we acquire this new information much faster. For instance, if a child learns to associate various odors with various locations around the home—such as associating the smell of bread with the kitchen and the smell air freshener with the bathroom—they will find it easier to make new odor-place associations outside the home, such as associating gasoline with gas stations.

It is thought that the de novo learning process is achieved when the newly acquired information is being consolidated and transferred into long-term memory storage within networks of neurons in the brain. The process of consolidation is believed to lead to the formation of mental schemas that can accelerate learning of novel but related information. Although the concepts of mental schema and related learning are widely used in psychology and education, their underlying neuronal mechanisms are poorly understood.

The formation of new memories depends on a part of the brain called the hippocampus and involves changes in the strength of the connectivity between groups of neurons in a process called synaptic plasticity. In particular, the interaction between a chemical called glutamate, which is released by sender neurons, and proteins called NMDA receptors (which bind the glutamate molecules) on receiver neurons have a central role in synaptic plasticity. Recently, based on experiments with rodents, it has been proposed that the hippocampus is also crucial for the formation of the mental schemas that can accelerate the learning of new spatial association tasks, such as the odor-place associations described above.

Now, Dragoi and Tonegawa reveal that the NMDA receptor in a key subregion of the hippocampus is also involved in the de novo learning of spatial tasks. Using repeated exposures to novel spatial experiences and genetic techniques to block the NMDA receptors in this subregion in mice, Dragoi and Tonegawa discovered that de novo learning involves synaptic plasticity in the hippocampus and, possibly, other regions of the brain. This de novo learning, in turn, enables subsequent spatial learning to be accelerated, even when the NMDA receptors are absent. These results reveal that de novo learning, and related learning processes such as accelerated learning, are underpinned by a number of different mechanisms in the brain, which could help explain why damage to the hippocampus prevents the formation of new memories while preserving other forms of memory and learning.

DOI:http://dx.doi.org/10.7554/eLife.01326.002

Long-term dynamics of CA1 hippocampal place codes

Via Ca²⁺-imaging in freely behaving mice that repeatedly explored a familiar environment, we tracked thousands of CA1 pyramidal cells' place fields over weeks. Place coding was dynamic, for each day the ensemble representation of this environment involved a unique subset of cells. Yet, cells within the ∼15–25% overlap between any two of these subsets retained the same place fields, which sufficed to preserve an accurate spatial representation across weeks.

α-Calcium calmodulin kinase II modulates the temporal structure of hippocampal bursting patterns

The alpha calcium calmodulin kinase II (α-CaMKII) is known to play a key role in CA1/CA3 synaptic plasticity, hippocampal place cell stability and spatial learning. Additionally, there is evidence from hippocampal electrophysiological slice studies that this kinase has a role in regulating ion channels that control neuronal excitability. Here, we report in vivo single unit studies, with α-CaMKII mutant mice, in which threonine 305 was replaced with an aspartate (α-CaMKII(T305D) mutants), that indicate that this kinase modulates spike patterns in hippocampal pyramidal neurons. Previous studies showed that α-CaMKII(T305D) mutants have abnormalities in both hippocampal LTP and hippocampal-dependent learning. We found that besides decreased place cell stability, which could be caused by their LTP impairments, the hippocampal CA1 spike patterns of α-CaMKII(T305D) mutants were profoundly abnormal. Although overall firing rate, and overall burst frequency were not significantly altered in these mutants, inter-burst intervals, mean number of intra-burst spikes, ratio of intra-burst spikes to total spikes, and mean intra-burst intervals were significantly altered. In particular, the intra burst intervals of place cells in α-CaMKII(T305D) mutants showed higher variability than controls. These results provide in vivo evidence that besides its well-known function in synaptic plasticity, α-CaMKII, and in particular its inhibitory phosphorylation at threonine 305, also have a role in shaping the temporal structure of hippocampal burst patterns. These results suggest that some of the molecular processes involved in acquiring information may also shape the patterns used to encode this information.

Single units in the medial prefrontal cortex with anxiety-related firing patterns are preferentially influenced by ventral hippocampal activity

The medial prefrontal cortex (mPFC) and ventral hippocampus (vHPC) functionally interact during innate anxiety tasks. To explore the consequences of this interaction, we examined task-related firing of single units from the mPFC of mice exploring standard and modified versions of the elevated plus maze (EPM), an innate anxiety paradigm. Hippocampal local field potentials (LFPs) were simultaneously monitored. The population of mPFC units distinguished between safe and aversive locations within the maze, regardless of the nature of the anxiogenic stimulus. Strikingly, mPFC units with stronger task-related activity were more strongly coupled to theta-frequency activity in the vHPC LFP. Lastly, task-related activity was inversely correlated with behavioral measures of anxiety. These results clarify the role of the vHPC-mPFC circuit in innate anxiety and underscore how specific inputs may be involved in the generation of behaviorally relevant neural activity within the mPFC.

Uncoupling the D1-N-methyl-D-aspartate (NMDA) receptor complex promotes NMDA-dependent long-term potentiation and working memory

BACKGROUND

Although dopamine D1 receptors are involved in working memory, how D1 receptors contribute to this process remains unclear. Numerous studies have shown that D1 receptors have extensive functional interaction with N-methyl-D-aspartate (NMDA) receptor. Our group previously demonstrated that D1 receptors were able to regulate NMDA receptor functions through direct protein-protein interactions involving the carboxyl terminals of D1 receptors and NMDA receptor NR1a and NR2A subunits respectively. In this study, we explored the effects of the D1-NR1 interaction on NMDA receptor-dependent long-term potentiation (LTP) and working memory by using the TAT-conjugated interfering peptide (TAT-D1-t2).

METHODS

Miniature excitatory postsynaptic currents are recorded in rat hippocampal primary cultures. Coimmunoprecipitation and calcium/calmodulin-dependent protein kinase II (CaMKII) activity are measured in hippocampal slices and hippocampal neurons under the specified experimental conditions, respectively. Working memory was assessed using a delayed match-to-place protocol in the Morris Water Maze following administration of the TAT-D1-t2 peptide.

RESULTS

Electrophysiology experiments showed that activation of D1 receptor upregulates NMDA receptor-mediated LTP in a CaMKII-dependent manner. Furthermore, D1 receptor agonist stimulation promotes the NR1-CaMKII coupling and enhances the CaMKII activity; and the D1 receptor-mediated effects can be blocked by the application of the TAT-D1-t2 peptide. Interestingly, animals injected with TAT-D1-t2 peptide exhibited significantly impaired working memory.

CONCLUSIONS

Our study showed a critical role of NMDA-D1 direct protein-protein interaction in NMDA receptor-mediated LTP and working memory and implicated the involvement of CaMKII in this process.

Intracellular dynamics of hippocampal place cells during virtual navigation

Hippocampal place cells encode spatial information in rate and temporal codes. To examine the mechanisms underlying hippocampal coding, we measured the intracellular dynamics of place cells by combining in vivo whole cell recordings with a virtual reality system. Head-restrained mice, running on a spherical treadmill, interacted with a computer-generated visual environment to perform spatial behaviors. Robust place cell activity was present during movement along a virtual linear track. From whole cell recordings, we identified three subthreshold signatures of place fields: (1) an asymmetric ramp-like depolarization of the baseline membrane potential; (2) an increase in the amplitude of intracellular theta oscillations; and, (3) a phase precession of the intracellular theta oscillation relative to the extracellularly-recorded theta rhythm. These intracellular dynamics underlie the primary features of place cell rate and temporal codes. The virtual reality system developed here will enable new experimental approaches to study the neural circuits underlying navigation.

Impaired long-term stability of CA1 place cell representation in mice lacking the transcription factor zif268/egr1

Zif268 is a transcriptional regulator that plays a crucial role in maintenance of the late phases of hippocampal long-term potentiation (LTP) and consolidation of spatial memories. Because the hippocampal place cell system is essential for long-term spatial memory, we tested the hypothesis that zif268 is required for long-term stability of hippocampal place cell representations by recording CA1 place cells in mice lacking zif268. We found that zif268 gene deletion destabilized the representation of a familiar environment after exposure to a novel environment and impaired the long-term (24 h), but not short-term (1 h), stability of newly formed representations. These impairments could be rescued by repeated exposure to the novel environment, however. These results indicate that zif268 contributes to the long-term stability of spatial representations in CA1 and support the notion that the long-term stability of place cell representations requires transcription-dependent mechanisms similar to those observed in LTP.

Place cell firing correlates with memory deficits and amyloid plaque burden in Tg2576 Alzheimer mouse model

Alzheimer's disease (AD) is associated with progressive memory decline. Hippocampal place cells are a well understood candidate for the neural basis of one type of memory in rodents; these cells identify the animal's location in an environment and are crucial for spatial memory and navigation. We have recorded place cells in the Tg2576 mouse model of AD, and we report that aged (16 mo) but not young (3 mo) transgenic mice show degraded neuronal representations of the environment. The level of place cell degradation correlates with the animals' (poorer) spatial memory as tested in a forced-choice spatial alternation T-maze task and with hippocampal, but not neocortical, amyloid plaque burden. Place cell recording provides a sensitive assay for measuring the amount and rate of functional deterioration in animal models of dementia as well as providing a quantifiable physiological indication of the beneficial effects of potential therapies.